Present day society relies on extensive use of automation for packaging products for sale. This usually entails a means for determining the quantity of material placed within a retail package. Weighing is the most practical and acceptable form for controlling the material placed in a package and this is accomplished largely by scales known as in-motion weighing devices or checkweighers. Because of the speeds involved, this requires very precise real time measurement of containers prior to entering the weighing/filling station and during the filling operation so that filling may be terminated at a precise point. Setting up such a system is critical, the slightest miscalculation will result in enormous losses by over filling packages or excessive customer complaints and lawsuits due to under-filled packages.
In the past, the very critical operation of setting up an in-motion weigher included the following steps usually accomplished manually with the aid of an assortment of calculating devices and test equipment. The steps include a first operation where the load of the platform sitting on top of the transducer device is calculated. This is commonly referred to as the dead load.
Once the dead load is established, a ranging function is set. Usually this is accomplished via a potentiometer and uses the highest weight package to set up the system. Alternately, the gain or range potentiometer is adjusted each time a different weight range is used.
Typical transducers used in such systems are a strain gauge load cell or a flexure type scale system using either a LVDT (Linear Variable Differential Transformer), a DCDT (Direct Current Differential Transducer), or force restoration (a magnetic means of counteracting forces in the scale by pumping current in the opposite direction to bring the scale back to an equilibrium point which must be offset).
Because the systems are electronic, they have some inherent noise imposed on control signals; typically, in-motion or checkweighing scales have noise in the range of 20 Hz or lower. This and higher values of noise have been filtered out by a low pass filter. However, the method by which this was done in the past used typical active analog filters and required quite a bit of human intervention with regard to using an oscilloscope, or strip chart recorder, to monitor the actual scale output and determine the appropriate resistor network or resistor values to tune the filter to provide a reasonable weight signal and, yet, eliminate higher frequency noises.
The recorder or oscilloscope was also required to determine the point at which sampling of the net weight signal was taken, and the window over which the weight signal was read was also a potentiometer type adjustment requiring human intervention. Furthermore, the filter value was dependent on the package speed and weight and, therefore, any time a speed or weight change was required, manual intervention and resetup were required. Analog filter values also limited the rate at which packages could be weighed due to charge and discharge time of the filters being used.
The potential errors and operator skill requirements of prior art systems are obvious and this has led to constantly increasing packaging costs and thus increase consumer costs.